1. Field of the Invention.
The invention generally relates to circularly polarized antennas. In particular, the invention relates to wide band, low cost, planar, circularly polarized microstrip antennas.
2. Discussion of the Background.
Mobile systems need circularly polarized radiation. When a transmitter and receiver are stationary, the transmitter and receiver can be aligned, so that only linearly polarized radiation is necessary. However, when an airplane is in flight, or a satellite is in orbit, the airplane or satellite will not be able to detect radiation from a transmitter if the receiving antenna is not aligned with the radiation. Therefore, many commercial and military systems need circularly polarized antennas. In addition to being circularly polarized, these antennas need to be compact, low cost, rugged, and have a wide bandwidth. The systems that use these antennas include aircraft and space systems, electronic support systems, and communications systems.
Presently, spiral antennas are used for producing wideband circularly polarized (CP) radiation. However, spiral antennas have limited applications, because the structure of spiral antennas has co-planar metallic strips. There are two problems with using co-planar metallic strips to form an antenna.
First, the radiation from the antenna is bi-directional because of the co-planar structure. Accordingly, the strong radiation to the back side of the antenna plane must be reduced significantly. The resultant structure with reduced back radiation is bulky, large, and has degraded performance. Second, the co-planar metallic strips reduce the power handling capacity of the antenna because of high fringe radiation fields at the feed junction to the antenna.
Clearly, spiral antennas meet the need of circular polarization with a wide bandwidth, but fail miserably in providing an antenna that is compact, low cost, rugged and conformal, with a high power handling capacity. Therefore, there is a long-felt need in this field for a circularly polarized antenna that does not have the bulky size, degraded performance, or reduced power handling capacity of spiral antennas, but instead is compact, low cost, rugged, and has a high power handling capacity.
Microstrip antennas are an excellent improvement over large and bulky antennas, such as co-planar spiral antennas. Microstrip antennas are lightweight and low cost, and have a low profile because they are planar. Microstrip antennas overcome many of the problems associated with bulky antennas. However, prior art microstrip antennas have long suffered from an inherently narrow bandwidth, typically less than 1% for circularly polarized radiation. Prior art resonant or standing wave microstrip antennas provide about a 3% bandwidth for a linearly polarized antennas and only about 1% bandwidth for circularly polarized antennas. This limitation makes microstrip antennas useless for applications that require wideband circularly polarized radiation. In circularly polarized radiation, a circularly polarized radiation within 6 dB of all polarizations, which is provided by this invention""s antenna, is considered a good CP frequency bandwidth.
The present invention overcomes the long-standing bandwidth limitations of prior art microstrip antennas by providing a circularly polarized leaky-wave wideband traveling wave antenna that provides better than an 11%, 6 dB frequency bandwidth with only a single RF input. These significant differences in bandwidth results are compared in TABLE I below.
TABLE I also shows other bandwidth results based on different dielectric constants, including this invention""s leaky wave wideband traveling-wave microstrip antenna achieving more than a 20% wide frequency bandwidth when materials with a lower dielectric constant of about 1.1 is employed instead of a 2.2 dielectric constant. TABLE I demonstrates that this invention""s dramatic bandwidth improvement can satisfy the strong need in this field for a microstrip antenna that can radiate wideband, circularly polarized radiation, and thereby overcome the drawbacks, disadvantages and limitations of prior art narrow bandwidth microstrip antennas.
This invention""s circularly polarized leaky-wave wideband traveling wave antenna also differs from prior art circularly polarized microstrip antennas that split the RF energy into two and use two separate SMA connectors, by advantageously employing an innovative single feed, which is more efficient than the split RF energy technique because it eliminates the extra circuitry needed for the splitting technique, and the associated weight and costs. The prior art splitting technique does not operate in wide bandwidth antennas, because a delay line producing the required 90 degrees phase shift for CP radiation at the highest frequency of the bandwidth might produce only 60 degrees at the lower frequency.
Not only does this invention provide a substantial improvement in bandwidth capacity, but the circularly polarized microstrip antenna of this invention is also significantly different from prior art microstrip antennas that are either circularly shaped or include a circular array of microstrip antennas. Further, this invention""s antenna is thin and its linear dimension is approximately equal to one wavelength, making it more compact than prior art wideband antennas. Additionally, the present invention provides a circularly polarized leaky-wave wideband traveling wave antenna based on the traveling wave principle, which is completely different from the standing wave technique utilized in prior art compact microstrip antennas.
The present invention provides a microstrip antenna that can produce wideband, circularly polarized radiation. The present invention, therefore, is a compact, low cost. rugged, conformal, planar, and circularly polarized microstrip antenna. The present invention eliminates the bulky size and reduced power handling capability found in spiral circularly polarized antennas. The present invention eliminates the narrow bandwidth that is inherent to microstrip antennas. The present invention combines the advantages of wideband circularly polarized radiation with the advantages of lightweight, low profile, low cost, and planar microstrip antennas. Compared with other leaky wave antennas, the present invention is planar and is easily implemented in an MMIC (monolithic microwave integrated circuit) environment.
The present invention is an antenna comprising means for producing leaky wave radiation and means for circularly polarizing the leaky wave radiation. In another embodiment, the invention is a method of producing a circularly polarized wide band traveling wave from a microstrip antenna. In a further embodiment, the invention is a circularly polarized wide band traveling wave formed by producing leaky wave radiation and circularly polarizing the leaky wave radiation.